One example of a drilling tool is proposed in each of the following documents.
Japanese Utility Model Publication No. Hei 6.18710 discloses a tapping screw having a drill cutting edge provided at the tip section and an end mill cutting edge formed next to the drill cutting edge. In this prior art, the tip section of the drill cutting edge is formed to cause the core section to protrude in the axial direction.
Japanese Utility Model Publication No. Hei 6-254711 discloses an end mill of which the tool body is provided with first and second flutes, wherein the first flute is provided with a first peripheral tip and a central tip, while the second flute is provided with a second peripheral tip. In this prior art, the cutting edge lines of a tip section of the first and second peripheral tips are formed to be depressed toward the tool core.
The difference between the drill and the end mill is now clarified. The drill has the cutting edge only on the tip and is used for drilling and chamfering operations. On the other hand, the end mill has an end cutting edge and a peripheral cutting edge and can be used for drilling, grooving and a side cutting operation, but a shallow hole can only be made because the tip room is narrow.
Japanese Utility Model Publication No. Hei 7-40118 discloses a drill of which the spiral angle of the cutting edge is gradually decreased toward a shank. In this prior art, it is shown that the tip edge line of the cutting edge is depressed.
FIG. 5 explains an example in which a deep hole is made in a cylinder block with a conventional drill (point angle: 118 degrees) of which the tip of the cutting edge is projecting. As shown in FIG. 5 (a), the cylinder block is formed with a lower hole (i.e., a mold-release hole) in the case of casting. If cutting proceeds in such a condition in which the center of the lower hole is not in line with the core of the drill, a difference is caused in the radial force even though a cutting force received from the cutting surface is the same as shown in FIG. 5 (b). As a result, a sudden hole-bending may be generated as shown in FIG. 5 (c).
On the other hand, when the tip section of the cutting edge is formed to have a depressed shape, the bending can be reduced because the tip section does not follow the lower hole in the case of cutting and the rectilinear propagation is good. However, bending is readily generated when the hole to be cut is deep because there is still a difference in the radial force.